There is no plugin API available to use with the test client currently. You cannot install BetterDiscord, Powercord/Replugged, or other client mod plugins without heavy modification at this time.

However, you can install Vencord quite easily with only slight modifications to the user script.

For instances you control, simply find and replace all mentions of GM_xmlhttpRequest with new XMLHttpRequest, and unsafeWindow with window. Throw the edited file into the assets/preload-plugins directory, restart the server, and you now have Vencord installed instance-wide!

If you want to install Vencord on instances you do not control, do not perform any of the above steps, and simply modify the @match line at the top to include the instance(s). For staging.fosscord.com:

// @match *://*.staging.fosscord.com/*
You may now install the modified user script in your favourite user script browser extension, such as [Tampermonkey](https://www.tampermonkey.net/)